Mitochondria are the primary sites of energy production in cells. Energy is produced by the oxidation of glucose and fatty acids. Glucose is initially oxidized to pyruvate in the cytoplasm. Fatty acids and pyruvate are transported to the mitochondria for complete oxidation to CO2 by coenzyme A (CoA). This oxidation is coupled by enzymes to the transport of electrons from NADH and FADH2 to oxygen and to the synthesis of ATP (oxidative phosphorylation) from ADP and Pi. ATP synthesis is carried out by the F0F1 ATPase complex in the mitochondrial inner membrane. ATP then provides the primary source of energy for driving a cell's many energy-requiring reactions.
Enzyme complexes responsible for electron transport and ATP synthesis include F0, ubiquinone (CoQ)-cytochrome c reductase, cytochrome b, cytochrome c1, FeS protein, and cytochrome c oxidase. ATP synthesis requires membrane transport enzymes including the phosphate transporter and the ATP-ADP antiport protein. The ATP-binding casette (ABC) superfamily of transport proteins is also observed in mitochondria (Hogue, D. L. et al. (1999) J. Mol. Biol. 285:379–389). Another mitochondrial transport enzyme is brown fat uncoupling protein, which dissipates oxidative energy as heat, and may be involved in the fever response to infection and trauma (Cannon, B. et al. (1998) Ann. NY Acad. Sci 856:171–187).
Electron carriers such as cytochromes accept electrons from NADH or FADH2 and donate them to other electron carriers. Most electron-transferring proteins, except ubiquinone, are prosthetic groups such as flavins, heme, FeS clusters, and copper, bound to inner membrane proteins. Adrenodoxin, for example, is an FeS protein that forms a complex with NADPH:adrenodoxin reductase and cytocbrome P450. Cytochromes contain a heme prosthetic group, a porphyrin ring containing a tightly bound iron atom. For a review of mitochondrial metabolism and regulation, see Lodish, H. et al. (1995) Molecular Cell Biology, Scientific American Books, New York N.Y., pp. 745–797.
The majority of mitochondrial proteins are encoded by nuclear genes, are synthesized on cytosolic ribosomes, and are imported into the mitochondria. Nuclear-encoded proteins which are destined for the mitochondrial matrix typically contain positively-charged amino terminal signal sequences. Import of these preproteins from the cytoplasm requires a multisubunit protein complex in the outer membrane known as the translocase of outer mitochondrial membrane (TOM; previously designated MOM; Pfanner, N. et al. (1996) Trends Biochem Sci. 21:51–52) and at least three inner membrane proteins which comprise the translocase of inner mitochondrial membrane (TIM; previously designated MIM; Pfanner et al., supra). An inside-negative membrane potential across the inner mitochondrial membrane is also required for preprotein import. Preproteins are recognized by surface receptor components of the TOM complex and are translocated through a proteinaceous pore formed by other TOM components. Proteins targeted to the matrix are then recognized by the import machinery of the TIM complex. The import systems of the outer and inner membranes can function independently (Segui-Real, B. et al. (1993) EMBO J. 12:2211–2218).
Once precursor proteins have entered the mitochondria, the leader peptide is cleaved by a signal peptidase to generate the mature protein. Most leader peptides are removed in a one step process by a protease termed mitochondrial processing peptidase (MPP) (Paces, V. et al. (1993) Proc. Natl. Acad. Sci. USA 90:5355–5358). In some cases a two-step process occurs in which MPP generates an intermediate precursor form which is cleaved by a second enzyme, mitochondrial intermediate peptidase, to generate the mature protein.
Mitochondrial dysfunction leads to impaired calcium buffering, generation of free radicals that may participate in deleterious intracellular and extracellular processes, changes in mitochondrial permeability, and oxidative damage which is observed in several neurodegenerative diseases. Neurodegenerative diseases linked to mitochondrial dysfunction include some forms of Alzheimer's disease, Friedreich's ataxia, familial amyotrophic lateral sclerosis, and Huntington's disease (Beal, M. F. (1998) Biochim. Biophys. Acta 1366:211–213). The myocardium is heavily dependent on oxidative metabolism, so mitochondrial dysfunction often leads to heart disease (DiMauro, S. and M. Hirano (1998) Curr. Opin. Cardiol. 13:190–197). Mitochondria are implicated in disorders of cell proliferation, since they play an important role in a cell's decision to proliferate or self-destruct through apoptosis. The oncoprotein Bcl-2, for example, promotes cell proliferation by stabilizing mitochondrial membranes so that apoptosis signals are not released (Susin, S. A. (1998) Biochim. Biophys. Acta 1366:151–165).
The discovery of new mitochondrial proteins and the polynucleotides encoding them satisfies a need in the art by providing new compositions which are useful in the diagnosis, prevention, and treatment of disorders of cell proliferation, inflammation, and immune response.